1. Technical Field of the Invention
The present invention relates generally to a fuel injector for internal combustion engines, and more particularly to an improved structure of a fuel injector for installation of a piezoelectric device used as a valve actuator of the fuel injector.
2. Background Art
Typical fuel injectors used in, for example, internal combustion diesel engines of automotive vehicles are designed to drive a three-way valve or a two-way valve connected to a common rail in which a high pressure fuel is stored for opening and closing a fuel supply passage selectively. When it is required to inject the fuel into the engine, the fuel injector changes the fuel pressure acting on a needle to lift up the needle for opening a spray hole to initiate the fuel injection.
As a valve actuator to open and close the three-way valve or the two-way valve, a solenoid valve has been usually used. In recent years, however, an attempt is made to utilize an piezoelectric device which expands or contracts in response to input of an electric signal to actuate a valve for controlling the fuel injection precisely. For example, a valve actuator is proposed which consists of a piezoelectric device made up of a stack of piezoelectric layers and a piston. In operation, the voltage is applied to the piezoelectric device. The piezoelectric device then contracts or expands to move the piston to open or close, for example, a three-way valve to control the back pressure of a nozzle needle of a fuel injector. The three-way valve works to switch communications between a back pressure chamber formed adjacent the nozzle needle and a high-pressure fuel path and between the back pressure chamber and a drain passage. When the back pressure chamber communicates with the fuel passage so that the pressure in the back pressure chamber drops, it will cause the nozzle needle to be lifted up to initiate a jet of fuel from a spray hole. Alternatively, when the back pressure chamber communicates with the high-pressure fuel passage, the fuel flows from the high-pressure fuel passage to the back pressure chamber, thereby moving the nozzle needle downward to close the spray hole.
The piezoelectric device is made by laminating the piezoelectric layers each having upper and lower surfaces on which electrodes are formed and applying a conductive paste to a side surface of the lamination to form side terminals which connect negative and positive sides of the electrodes, respectively. Installation of the piezoelectric device in a housing is accomplished by coupling the side terminals to a connector through leads, fitting an insulator tube on the periphery of the piezoelectric device, and inserting it into a vertical chamber of the housing. After the installation of the piezoelectric device, a hermetic seal is formed by placing the whole of the housing in a mold and forcing resin into the mold to seal an upper end of the housing.
The piezoelectric device is usually made from PZT (lead zirconate titanate). The PZT contains the lead that is a harmful substance and thus needs to be withdrawn after the piezoelectric device is used up. The withdrawal of the lead requires cutting the housing because the upper end of the housing is, as described above, sealed by resin. It is, thus, quite inconvenient. Further, there is a problem that parts cannot be removed from the housing after assembly thereof, therefore, it is impossible to replace the parts and adjust characteristics of the fuel injection finely.
The piezoelectric device, the insulator tube, and the connector are not secured completely during assembly thereof and thus are not easy to handle, which may lead to the breakage of the insulator tube. The connector is covered with a resin material using a mold after the fu(:l injector is assembled to insulate the connector from the injector body and thus is fixed in orientation thereof in a circumferential direction of the injector. Accordingly, it is necessary to prepare a connector mold for every type of engine, resulting in an increase in manufacturing cost of the injector.
Japanese Patent No. 3010835 discloses a piezoelectric device which is disposed hermetically within a casing which has a bellows for avoiding the ingress of moisture or foreign objects into the piezoelectric device. This structure, however, has the drawbacks in that the bellows has a larger diameter and is difficult to install in small-sized fuel injectors. If the size of the piezoelectric device is decreased to match with that of the fuel injectors, it may cause the performance thereof to be reduced. For theses reasons, fuel injectors equipped with the piezoelectric device as an actuator are not yet put into practical use.